Method and apparatus to counterbalance intrinsic positive end expiratory pressure

ABSTRACT

The invention prevents dynamic airway compression during ventilatory support of a patient. The respiratory airflow is determined by measurement or calculation, and a measure of the degree of dynamic airway compression is derived from the determined airflow. This measure is servo-controlled to be zero by increasing expiratory pressure if the measure of the degree of dynamic airway compression is large or increasing, and by reducing expiratory pressure if the measure of the degree of dynamic airway compression is small or zero.

[0001] This application claims the benefit of U.S. provisionalapplication No. 60/116,121, filed Jan. 15, 1999.

FIELD OF THE INVENTION

[0002] This invention pertains to the field of ventilatory support forrespiratory failure, particularly due to lung disease, and in particularto automatically providing sufficient end expiratory pressure to unloadintrinsic positive end expiratory pressure (PEEPi).

BACKGROUND OF THE INVENTION

[0003] Subjects with chronic airflow limitation (CAL) due, for example,to emphysema and chronic bronchitis, may require ventilatory assistance,particularly during periods of acute exacerbation, or routinely atnight.

[0004] Ventilatory support can reduce the work of breathing, reduce thesensation of breathlessness, and improve blood gases (oxygen and carbondioxide levels). In subjects with CAL, most of the work of breathing isdue to the high airway resistance. Approximately two thirds of thisresistance is relatively fixed, and due to narrowing of the airways.However, of the order of one third of the resistance is due to dynamicairway compression during expiration. Dynamic airway compression occurswhen the pleural pressure exceeds the pressure in the lumen of theairway during expiration, causing flow to become independent of effort.

[0005] In a normal subject, the alveolar pressure decays exponentiallyduring expiration, so that the expiratory flow and alveolar pressure(relative to atmospheric) are both approximately zero at the end ofexpiration, and the lungs and chest wall have returned to their passiveequilibrium volume V_(R). In patients with CAL, however, as a result ofdynamic airway compression and fixed reduced expiratory flow rate, it isnot possible for the lungs to return to V_(R) in the time allowed beforethe start of the next inspiration. The chest is hyperinflated. Thealveolar pressure remains positive, on the order of 5 to 15 cmH₂O at theend of expiration. This raised alveolar pressure is termed intrinsicpositive end expiratory pressure, or PEEPi. (Other names for thephenomenon are covert PEEP and occult PEEP.)

[0006] An important effect of the hyperinflation is that the patientmust overcome the elastic recoil of the hyperinflated chest wall beforeinspiratory airflow can commence. The PEEPi is said to act as aninspiratory threshold load. A further undesirable effect of PEEPi isthat during artificial mechanical ventilatory support, it interferessubstantially with the triggering of the ventilator, causingpatient-machine asynchrony.

[0007] It is now well understood that the addition of a counterbalancingexternal positive end expiratory pressure (called external PEEP, or justPEEP), approximately equal in magnitude to PEEPi, is of great benefit.First, it prevents dynamic airway compression, permitting greaterexpiratory airflow. Second, it balances the inspiratory threshold load.Third, it improves triggering of a ventilator by the patient.

[0008] Use of excessive PEEP, however, can be disadvantageous and evendangerous. Excessive PEEP above and beyond PEEPi will cause yet furtherhyperinflation. This will result in stiffening of the lung and chestwall, and an increase in the elastic work of breathing. It will alsocause reduced cardiac output, and can lead to barotrauma. Further, thepeak inspiratory airway pressure during ventilatory support cannot bearbitrarily increased without either exceeding the capacity of theventilator, or reaching a pressure that is itself dangerous. Finally,excessive external PEEP will also reduce the possible airway pressureexcursion or headroom available for lung inflation.

[0009] Therefore, it is advisable when applying external PEEP to set theexternal PEEP as close as possible to PEEPi. Since PEEPi varies fromtime to time, depending on a number of factors including, for example,the resistance of the small airways and the respiratory rate, both ofwhich change with changing sleep stage, chest infection, orbronchospasm, it is desirable to be able to make multiple, or evencontinuous, measurements of PEEPi in order to optimize external PEEP.

[0010] A typical patient in an intensive care unit is heavily sedatedand paralyzed during ventilatory support, and it is straightforward tomeasure the PEEPi. It is necessary only to occlude the airway duringlate expiration, and measure the airway pressure, which, after a fewseconds of equilibration, will equal static PEEPi. Since the lung injuryin CAL is usually markedly heterogeneous, different alveoli will havedifferent end expiratory pressures, and static PEEPi is therefore aweighted average across all alveoli.

[0011] Another known method which is suitable for use in the paralyzedsedated patient is to measure the airway pressure at the start ofmachine inspiratory effort, and again at the start of actual inspiratoryairflow. The difference between these two pressures is the dynamicPEEPi. Dynamic PEEPi reflects the end expiratory pressure in the leastabnormal lung units, and substantially underestimates static PEEPi.

[0012] These simple methods do not work for patients who are not sedatedand paralyzed, and who are making spontaneous breathing efforts, becausethey do not take into account the patents' own respiratory muscleefforts.

[0013] One known method that is used with such patients requires aMuller manoeuvre (maximal inspiratory effort) during catheterization ofthe oesophagus and stomach, and is therefore completely unsatisfactoryfor repeated or continuous measurements in the ambulatory patient or thepatient who is being treated at home long-term.

[0014] Methods for measuring the airway conductance in spontaneouslybreathing patients using oscillometry are taught by Peslin et al.,Respiratory Mechanics Studied by Forced Oscillations During MechanicalVentilation, Eur Respir J 1993; 6:772-784, and by Farre et al., ServoControlled Generator to Measure Respiratory Impedance from 0.25 to 26 Hzin Ventilated Patients at Different PEEP Levels, Eur Respir J 1995;8:1222-1227. These references contemplate separate measurements forinspiration and expiration. Oscillometry requires modulation of theairway pressure at a high frequency, such as 4 Hz, and measurement ofthe resultant modulation of the respiratory airflow at that frequency.However, these references fail to describe servo-controlling ofventilation to increase or decrease PEEP so that the inspiratory aridexpiratory conductances are approximately equal.

[0015] Oscillometry has been used to control nasal CPAP (see U.S. Pat.No. 5,617,846) or bilevel CPAP for the treatment of obstructive sleepapnea (see U.S. Pat. No. 5,458,137). The problem there is essentiallyopposite to the problem under consideration here. In obstructive sleepapnea, there is increased resistance during inspiration, and the abovetwo patents teach that increased resistance during inspiration can betreated by an increase in pressure. In patients with CAL and dynamicairway compression, there is increased resistance during expiration.

[0016] There is no known method or apparatus which can automatically orcontinuously control a ventilator or CPAP apparatus in consciousspontaneously breathing patients in order to prevent expiratory airflowlimitation or to unload PEEPi in CAL.

[0017] Yet another known method for estimating PEEPi, taught, forexample, by Rossi et al., The Role of PEEP in Patients with ChronicObstructive Pulmonary Disease during Assisted Ventilation, Eur Respir J1990; 3:818-822, is to examine the shape of the expiratory flow-volumecurve, which has been observed to be exponential, if there is no dynamicairway compression. The reference further notes that in the absence ofPEEPi, the flow-volume curve becomes a straight line.

[0018] The above known art only contemplates the application of anexternal pressure which is constant during any one expiratory cycle.However, the elastic recoil of the lung is higher at high lung volume,and lower at low lung volume. Therefore, it may be advantageous to findthe minimum external pressure at each moment in time during anexpiration that will prevent dynamic airway compression during thatexpiration.

[0019] It is an object of our invention to vary the ventilatory pressureduring expiration as a function of the degree of the patient's dynamicairway compression.

[0020] It is another object of our invention to vary the ventilatorypressure automatically based solely on continuous measurements that arealready taken in conventional CPAP and ventilator apparatuses.

SUMMARY OF THE INVENTION

[0021] The present invention seeks to provide continuous and automaticadjustment of the expiratory pressure during ventilatory support, so asto substantially prevent dynamic airway compression and unload intrinsicPEEP with the smallest amount of external expiratory pressure.

[0022] The basic method of the invention prevents dynamic airwaycompression during ventilatory support using a conventional interface toa patient's airway such as a face mask, nose mask, or endotracheal ortracheotomy tube, and providing the interface with an exhaust and asupply of breathable gas at a variable pressure as is known in the CPAPand ventilatory arts. The respiratory airflow is determined bymeasurement or calculation, and a measure of the degree of dynamicairway compression is derived. This measure is servo-controlled,preferably to be zero, by increasing expiratory pressure if the measureof the degree of dynamic airway compression is large or increasing, andby reducing expiratory pressure if the measure of the degree of dynamicairway compression is small or zero.

[0023] The measure of the degree of dynamic airway compression may be aninstantaneous or pointwise measure within any given breath, and the stepof servo-controlling the measure to be zero may similarly be performedpointwise within a given breath, so that the expiratory pressure issimilarly varied pointwise within a breath. As an alternative to thusbasing the airway compression determination and the servo control onmultiple airflow determinations made within each individual respiratorycycle, the derivation of the measure of the degree of dynamic airwaycompression and the servo-controlling of the airway compression may beperformed across a plurality of respiratory cycles.

[0024] During expiration, the expiratory pressure increase may be linearas a function of expired volume as will be described below.

[0025] The measure of the degree of dynamic airway compression ispreferably derived by measuring the airway conductance separately duringthe inspiratory and expiratory portions of one or more respiratorycycles, and calculating the measure of the degree of dynamic airwayconductance as a function of the inspiratory conductance minus theexpiratory conductance, or alternatively as the ratio of the inspiratoryconductance to the expiratory conductance. The two separate conductancesduring inspiration and expiration may be measured by superimposing ahigh-frequency oscillation on the patient interface pressure, at a knownor measured amplitude, identifying the inspiratory and expiratoryportions of each respiratory cycle, measuring the component of therespiratory airflow at the high frequency separately over theinspiratory and expiratory portions of one or more respiratory cycles,and from these measurements and the determined pressure amplitudecalculating the inspiratory airway conductance and the expiratory airwayconductance.

[0026] Alternatively, the measure of the degree of dynamic compressionmay be derived from the shape of the expiratory airflow versus timecurve. The measure is zero when the expiratory flow decays exponentiallyfrom the moment of the peak expiratory flow to end expiration, but islarge when the expiratory flow decreases suddenly from the peakexpiratory flow and is then steady but non-zero for the remainder ofexpiration. The measure may be the ratio of the mean expiratory flowduring approximately the last 25% of expiratory time to the peakexpiratory flow.

[0027] Further objects, features and advantages of the invention willbecome apparent upon consideration of the following detailed descriptionin conjunction with the drawing which depicts illustrative apparatus forimplementing the method of our invention.

DETAILED DESCRIPTION OF THE INVENTION

[0028] In the drawing, a blower 1 supplies breathable gas to a mask 2 incommunication with a patient's airway via a delivery tube 3 andexhausted via an exhaust 4. Airflow at the mask 2 is measured using apneumotachograph 5 and a differential pressure transducer 6. The maskflow signal from the transducer 6 is then sampled by a microprocessor 7.Mask pressure is measured at the port 8 using a pressure transducer 9.The pressure signal from the transducer 6 is then sampled by themicroprocessor 7. The microprocessor 7 sends an instantaneous maskpressure request (i.e., desired) signal to a servo 10, which comparesthe pressure request signal with the actual pressure signal from thetransducer 9 to control a fan motor 11. Microprocessor settings can beadjusted via a serial port 12.

[0029] It is to be understood that the mask could equally be replacedwith a tracheotomy tube, endotracheal tube, nasal pillows, or othermeans of making a sealed connection between the air delivery means andthe subject's airway.

[0030] The invention involves the steps performed by the microprocessorto determine the desired mask pressure. The microprocessor accepts themask airflow and pressure signals, and from these signals determines theinstantaneous flow through any leak between the mask and patient, by anyconvenient method. For example, the conductance of the leak may beestimated as the instantaneous mask airflow, low-pass filtered with atime constant of 10 seconds, divided by the similarly low-pass filteredsquare root of the instantaneous mask pressure, and the instantaneousleakage flow may then be calculated as the conductance multiplied by thesquare root of the instantaneous mask pressure. Respiratory airflow isthen calculated as the instantaneous mask airflow minus theinstantaneous leakage flow.

[0031] In the simple case of no intrinsic PEEP, the instantaneouspressure at the mask may be simply set as follows, in order to provideventilatory support to the patient:

P=P_(INSP) flow>0 (inspiration)

P=P_(EXP) flow<=0 (expiration)

[0032] where P_(EXP) is less than or equal to P_(INSP). Typically,P_(EXP) might be zero, and P_(INSP) might be of the order of 10 to 20cmH₂O.

[0033] Two embodiments for deriving a measure of the degree ofexpiratory airflow limitation will now be considered. In the firstembodiment, airway conductance during inspiration is compared withairway conductance during expiration, and a higher conductance duringinspiration indicates expiratory airflow limitation. Airway conductanceis calculated by superimposing on the instantaneous mask pressure a 4-Hzoscillation of amplitude 1 cmH₂O, and measuring the component of therespiratory airflow signal at 4 Hz. The conductance may be calculatedonce for each half cycle of the 4-Hz oscillation. In order to identifyinspiratory and expiratory halves of the respiratory cycle, therespiratory airflow is low-pass filtered to minimize the imposed 4-Hzoscillation, for example, by averaging measured respiratory airflow overa moving window of length 0.25 seconds. If the 4-Hz low-pass filteredflow is above a threshold such as 0.1 L/sec, it is taken to be theinspiratory half-cycle. Otherwise, it is taken as being the expiratoryhalf-cycle.

[0034] Conductance over one or more inspiratory half-cycles, and overone or more expiratory half cycles is now calculated, using standardaveraging or filtering techniques. The conductance during inspirationminus the conductance during expiration yields a first measure M₁ of thedegree of dynamic airway compression. Preferably, M₁ can be normalizedby dividing by the mean conductance over the entire breath or breaths,and a threshold value, for example, 0.2, can be subtracted so that onlydifferences in conductance of 20% or more are regarded as indicative ofdynamic airway compression. Thus, M₁=(average conductance duringinspiration−average conductance during expiration)/(average conductanceover entire breath)−0.2.

[0035] Finally, it is necessary to adjust the expiratory pressure toservo-control the difference in conductance to be zero. This can be donefor, example, by increasing P_(EXP) by (0.1)(M₁) cmH₂O per second. Usingthis method, if there is dynamic airway compression, P_(EXP) will slowlyincrease until M₁ reaches zero, at which point there will be no furtherdynamic airway compression. Changes in the pressure required to preventdynamic compression with the passage of time can be tracked. In anelaboration of this first embodiment, M₁ can be calculated as a functionof the time into expiration, and the pressure at different points intoexpiration servo-controlled separately within a breath.

[0036] In the second embodiment for deriving a measure of the degree ofexpiratory airflow limitation, the degree of expiratory flow limitationis calculated from the shape of the expiratory flow versus time curve.The expiratory portion of each breath is identified, for example, bytaking expiration as the period where airflow is less than 0.1 L/sec.The mean expiratory airflow during the final 25% of expiratory durationis calculated, and divided by the peak expiratory airflow. For a subjectwithout expiratory airflow limitation, this ratio will be close to zero,and less than a threshold such as 0.2, whereas for a subject withexpiratory airflow limitation, it will be larger, for example, in therange 0.2 to 0.6, with higher values indicating more severe dynamicairway compression. Therefore, a second measure of the degree ofexpiratory airflow limitation is M₂=(mean expiratory flow during last25% of expiratory time)/(peak expiratory flow)−threshold, where thethreshold is, for example, 0.2.

[0037] In the final step in this second embodiment, if M₂ is positive,the expiratory pressure P_(EXP) is increased slightly, for example by(0.1)(M₂) cmH₂O per breath. Conversely, if M₂ is negative, P_(EXP) isdecreased slightly, for example, by (0.1)(M₂) cmH₂O per breath.

[0038] A third embodiment, which can be used as an enhancement of theservo-controlling step in either of the above two embodiments, takesaccount of the fact that there is no dynamic compression at the start ofexpiration, and no external pressure is required to prevent dynamiccompression at the start of expiration, but that dynamic compressiondevelops as the elastic recoil decreases. Since the elastic recoilpressure decreases approximately linearly on expired volume, theexternal pressure required to be applied will increase approximatelylinearly as a function of expired volume. Therefore, in this thirdembodiment, expiratory pressure is set as:

P _(EXP)(t)=K V(t)/V _(T)

[0039] where P_(EXP)(t) is the pressure at time t in the expiratoryportion of a respiratory cycle, V(t) is the expired volume at time tinto the expiration, and V_(T) is the tidal volume of the previousinspiration. Thus, V(t)/V_(T) increases from 0 to 1 during expiration.The constant K is adjusted in order to servo-control either M₁ or M₂ tobe zero, and will approximate PEEPi.

[0040] Although the invention has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the application of the principles of theinvention. Numerous modifications may be made therein and otherarrangements may be devised without departing from the spirit and scopeof the invention.

What we claim is:
 1. A method for preventing dynamic airway compressionduring ventilatory support of a patient, comprising the steps of:providing an interface to the patient's airway, providing said interfacewith a supply of breathable gas at a controllable pressure and with anexhaust, determining respiratory airflow by one of measurement andcalculation, varying the pressure at the patient interface so as toprovide ventilatory support to the patient, from at least the determinedrespiratory airflow, deriving a measure of the degree of dynamic airwaycompression, and servo-controlling the measure of the degree of dynamicairway compression by increasing expiratory pressure if said measure islarge or increasing, and reducing expiratory pressure if said measure issmall or zero.
 2. The method as in claim 1 in which said measure of thedegree of dynamic airway compression and said servo-controlling of thedegree of dynamic airway compression are both based upon multiplerespiratory airflow determinations made within each individualrespiratory cycle
 3. The method as in claim 1 in which said measure ofthe degree of dynamic airway compression and said servo-controlling ofthe degree of dynamic airway compression are both based upon arespiratory airflow determination made across a plurality of respiratorycycles.
 4. The method as in claim 1 in which, during expiration,expiratory pressure is increased approximately linearly as a function ofexpired volume.
 5. The method as in claim 1 in which said measure of thedegree of dynamic airway compression is derived in accordance with thesub-steps of: measuring the patient's airway conductance separatelyduring the inspiratory and expiratory portions of at least onerespiratory cycle, and calculating the measure of the degree of dynamicairway conductance as a function of the values of the inspiratoryconductance and the expiratory conductance.
 6. The method as in claim 5in which the measure of the degree of dynamic airway conductance iscalculated as a function of the difference between the inspiratoryconductance and the expiratory conductance.
 7. The method as in claim 5in which the measure of the degree of dynamic airway conductance iscalculated as a function of the ratio of the inspiratory conductance tothe expiratory conductance.
 8. The method as in claim 5 in which theseparate conductances during inspiration and expiration are measured inaccordance with the sub-steps of: superimposing a high-frequencyoscillation on the patient interface pressure at a determined amplitude,identifying the inspiratory and expiratory portions of each respiratorycycle, measuring the component of the respiratory airflow at said highfrequency, separately over the inspiratory and expiratory portions of atleast one respiratory cycle, and from the high-frequency componentairflow measurements and the determined pressure amplitude, calculatingthe inspiratory airway conductance and the expiratory airwayconductance.
 9. The method as in claim 1 in which said measure of thedegree of dynamic airway compression is derived from the shape of theexpiratory airflow versus time curve.
 10. The method as in claim 9 inwhich said measure is zero when the expiratory airflow decaysexponentially from the moment of the peak expiratory airflow to endexpiration, but is large when the expiratory airflow decreases suddenlyfrom the peak expiratory flow and is then steady but non-zero for theremainder of expiration.
 11. The method as in claim 10 in which saidmeasure is the ratio of the mean expiratory airflow during approximatelythe last 25% of expiratory time to the peak expiratory flow.
 12. Amethod for preventing dynamic airway compression during ventilatorysupport of a patient, comprising the steps of: providing a variablepressure to the patient's airway so as to provide ventilatory support,determining the patient's respiratory airflow, deriving a measure of thedegree of dynamic airway compression as a function of the determinedrespiratory airflow, and increasing or decreasing expiratory pressure inaccordance with the derived measure of the degree of dynamic airwaycompression.
 13. The method as in claim 12 in which the expiratorypressure is increased or decreased by servo-controlling the measure ofthe degree of dynamic airway compression.
 14. The method as in claim 13in which expiratory pressure is increased if said measure of the degreeof dynamic airway compression is large or increasing, and expiratorypressure is reduced if said measure of the degree of dynamic airwaycompression is small or zero.
 15. The method as in claim 12 in whichexpiratory pressure is increased if said measure of the degree ofdynamic airway compression is large or increasing, and expiratorypressure is reduced if said measure of the degree of dynamic airwaycompression is small or zero.
 16. The method as in claim 12 in whichsaid measure of the degree of dynamic airway compression is based upon arespiratory airflow determination made across a plurality of respiratorycycles.
 17. The method as in claim 12 in which said measure of thedegree of dynamic airway compression is derived in accordance with thesub-steps of: measuring the patient's airway conductance separatelyduring the inspiratory and expiratory portions of at least onerespiratory cycle, and calculating the measure of the degree of dynamicairway conductance as a function of the values of the inspiratoryconductance and the expiratory conductance.
 18. The method as in claim17 in which the measure of the degree of dynamic airway conductance iscalculated as a function of the difference between the inspiratoryconductance and the expiratory conductance.
 19. The method as in claim17 in which the measure of the degree of dynamic airway conductance iscalculated as a function of the ratio of the inspiratory conductance tothe expiratory conductance.
 20. The method as in claim 17 in which theseparate conductances during inspiration and expiration are measured inaccordance with the sub-steps of: superimposing a high-frequencyoscillation on the variable pressure provided to the patient's airway,identifying the inspiratory and expiratory portions of each respiratorycycle, measuring the component of the respiratory airflow at said highfrequency, separately over the inspiratory and expiratory portions of atleast one respiratory cycle, and calculating the inspiratory andexpiratory airway conductances from the high-frequency component airflowmeasurements.
 21. The method as in claim 12 in which said measure of thedegree of dynamic airway compression is derived from the shape of theexpiratory airflow versus time curve.
 22. The method as in claim 21 inwhich said measure is zero when the expiratory airflow decaysexponentially from the moment of the peak expiratory airflow to endexpiration, but is large when the expiratory airflow decreases suddenlyfrom the peak expiratory flow and is then steady but non-zero for theremainder of expiration.